It has been publicly known in the art that catalysts containing precious metals such as Pt, Rh, Pd and Ir may efficiently purify exhaust gases. These precious metals are typically supported on Al2O3 having high surface area. The precious metals are supported on Al2O3 in a highly dispersed state, which is optimal for catalytic performance. Specifically, one precious metal supported on Al2O3, or combinations of a plurality of precious metals supported on Al2O3, for example, are utilized as catalysts for exhaust gas purification.
Incidentally, the precious metals tend to aggregate, reducing the number of active sites, due to extended exposure to high-temperature under severe conditions such as those of automobiles, thereby significantly decreasing their activity and effectiveness in exhaust gas purification. As such, catalysts with high precious metal content are utilized in order to purify automobile exhaust gases, which requires extremely high durability. However, catalysts with high precious metal content are remarkably expensive and, therefore, are undesirable from the viewpoint of developing inexpensive and highly active catalysts.
To counter this, use of precious metals in a form of complex oxide containing the precious metals and other elements has been proposed so as to avoid the agglomeration of the precious metals. With respect to Pd in particular, technologies for complex oxides of Pd and rare earth or alkaline metal elements have been disclosed (see Japanese Unexamined Patent Publication Nos. 61-209045, 01-43347, 04-27433, 04-341343 and 07-88372).
In addition, Pd supported on catalyst carriers such as Al2O3 exists in the state of PdO, which has high activity for exhaust gas purification; however, it changes into Pd metal upon extended exposure to high-temperatures of 900 degrees C. or higher; since Al2O3 is a stable compound and is far from interaction with the Pd supported thereon, the decomposition into Pd cannot be suppressed. Therefore, conventional catalysts having precious metals supported on carriers such as Al2O3 are inadequate with respect to their durability.
On the other hand, perovskite-type complex oxides have been developed in recent years for catalysts to purify exhaust gases. The perovskite-type complex oxides may be various combinations of elements, thus exhibiting an extremely wide variety of properties. It is publicly known that the precious metals supported on such complex oxides are significantly affected by the wide variety of properties of the complex oxides and that the properties of the precious metals change considerably.
A technology on the basis of these properties has been disclosed, where a portion of the precious metal content is incorporated into a crystal structure of perovskite-type (see Japanese Unexamined Patent Publication No. 06-100319). It is therein described that the precious metal incorporated into the crystal lattice may enhance the catalytic activity of the precious metal; this is because the precious metal particles are micronized and highly dispersed, and also due to lattice defects, which contribute to the catalytic activity, being generated moderately.
A catalyst for exhaust gas purification is also proposed, where A-site defect perovskite is used as a carrier in place of the conventionally used Al2O3 (see Japanese Unexamined Patent Publication No. 2003-175337). The catalyst for exhaust gas purification may prevent the reduction of PdO, which is an active species in the NO reduction reaction, into Pd, which has lower activity.
A technology by use of perovskite-type complex oxides has also been disclosed, where the durability of catalysts for exhaust gas purification is enhanced (see Japanese Unexamined Patent Publication No. 2004-41866). The durability of catalysts for exhaust gas purification may be enhanced by way of solid-dissolving Pd into a perovskite-type complex oxide, thereby suppressing Pd agglomeration.
However, the perovskite-type complex oxides are very highly crystalline complex oxides, making it impossible for the Pd inside the complex oxides to contribute to reaction. Therefore, the Pd is far from fully exhibiting its exhaust gas-purifying function, and its catalytic activity in purifying exhaust gases is not high.
A technology concerning a heat-resistant catalyst for exhaust gas purification has also been disclosed (see Japanese Unexamined Patent Publication No. 10-277393). The catalyst for exhaust gas purification has both a Pd complex oxide, containing at least one of the rare earth metals and alkaline earth metals, and a complex oxide containing at least one of the transition metals in a solid-solution or mixed condition, and exhibits higher heat resistance.
However, Pd in a solid-solution of the complex oxide containing at least one of the rare earth metals cannot contribute to reaction. Therefore, the Pd is far from fully exhibiting its exhaust gas-purifying function, and its catalytic activity in purifying exhaust gases is not high.
The present invention has been made in view of the problems described above; it is an object of the present invention to provide a catalyst for exhaust gas purification that can not only efficiently and simultaneously purify nitrogen oxides (NOx), hydrocarbons (HC) and carbon monoxide (CO) in exhaust gas emitted from internal combustion engines such as those of automobiles at lower temperatures, but can also exhibit higher durability.